Chapter 4. Threads and Processes

Although all programs use threads and processes, Red Hat Enterprise Linux for Real Time handles them in a different way to standard Red Hat Enterprise Linux. This chapter explains the Red Hat Enterprise Linux for Real Time approach to threads and processes.
Each CPU core is limited in the amount of work it can handle. To achieve greater efficiency, applications can execute different tasks simultaneously on multiple cores. This is called parallelizing.
Programs can be parallelized using threads. However, threads and processes are often confused, so it is important to understand the differences in the terms.
Process
A UNIX®-style process is an operating system construct that contains:
  1. Address mappings for virtual memory
  2. An execution context (PC, stack, registers)
  3. State/Accounting information
Linux processes started as exactly this style of process. When the concept of more than one process running inside one address space was developed, Linux turned to a process structure that shares an address space with another process. This works well, as long as the process data structure is kept small. For the remainder of this document, the term process refers to an independent address space, potentially containing multiple threads.
Thread
Strictly, a thread is a schedulable entity that contains:
  1. A program counter (PC)
  2. A register context
  3. A stack pointer
Multiple threads can exist within a process.
When programming on a Red Hat Enterprise Linux for Real Time system, there are two potential ways to parallelize the programs.
  1. Use the fork and exec functions to create new processes
  2. Use the Posix Threads (pthreads) API to create new threads within an already running process

Note

Evaluate how the components will interact before deciding how to parallelize them. If the components are independent of one another and will not interact very much or at all then creating a new address space and running as a new process is usually the better option. If, however, the components will need to share data or communicate frequently, running them as threads within one address space will usually be more efficient.

Note

For more information, or for further reading, the following man pages and books are related to the information given in this section:
  • fork(2)
  • exec(2)
  • Programming with POSIX Threads, David R. Butenhof, Addison-Wesley, ISBN 0-201-63392-2
  • Advanced Programming in the UNIX Environment, 2nd Ed., W. Richard Stevens and Stephen A. Rago, Addison-Wesley, ISBN 0-201-43307-9
  • “POSIX Threads Programming”, Blaise Barney, Lawrence Livermore National Laboratory, http://www.llnl.gov/computing/tutorials/pthreads/